Parallel robots are generally made up of a fixed support or base plate on which there are assembled actuators which, through articulated arms or kinematic links, are linked to a mobile platform which the corresponding tool is anchored to.
A robot of this type with three degrees of freedom is described, for example in U.S. Pat. No. 4,976,582, whereas patent EP 1 084 802 describes a parallel robot with four degrees of freedom (three translations and one rotation).
Parallel robots with four degrees of freedom are very appropriate for carrying out part handling tasks, packaging and assembly, and they have advantages over more complex parallel robots, for example with 6 degrees of freedom, in reference to simplicity, price and movement speed.
The object of European patent 1 084 802 is a parallel robot in which all the linkages of the mobile platform are articulation-type linkages, this being a favorable factor for reaching high speeds and accelerations. This robot comprises four kinematic links operated by means of four actuators which are fixed at one of their ends and with a specific orientation to the base plate, and at their other end to the kinematic link. In turn, the kinematic links are linked to the mobile platform carrying the tool, the mobile platform being made up of two parts or bars which are linked at their ends to the four kinematic links and a third bar which is assembled between the first two, through two articulated linkages, this bar being the one carrying the tool.
The mobile platform allows a rotation of ±45° which can be amplified by means of using gear rings.
The specific design of the mobile platform and the use of articulated linkages leads to obtaining an isostatic configuration and forces a non-homogenous arrangement of the kinematic links, and therefore of the actuators. This particular arrangement of the actuators combined with the isostatic configuration prevents the robot from having a homogenous performance and high rigidity in the entire working volume, which is an important disadvantage.
A modified configuration of the robot described in patent EP 1 084 802 is described for example in the following publication: “I4: A new parallel mechanism for SCARA motions” Proc. of IEEE ICRA: Int. Conf. on Robotics and Automation, Taipei, Taiwan, Sep. 14-19, 2003.
The configuration described in this article seeks to compensate the main limitation of patent EP 1 084 802, i.e. the non-homogenous performance and low rigidity that the configuration has in the entire working volume.
This new configuration is essentially based on a new mobile platform design, which must solve the aforementioned problems, replacing articulated linkages with prismatic joints and the gear rings with pinions-rack. These changes imply, firstly, a considerable reduction of the risk of collision between the different parts of the mobile platform.
Specifically, the mobile platform is formed by three parts, two side parts and a central part, the side parts being linked at their ends to the four kinematic links through ball joints, as in patent EP 1 084 802, whereas the central part is linked to the side parts by means of a prismatic joint. The four kinematic links are in turn fixed to the fixed platform through the four corresponding actuators.
In this configuration, as in European patent 1 084 802, the rotational motors can be replaced with linear motors and the articulated linkages with universal joints. In the specific case of the new configuration described in the mentioned article, four linear motors arranged in the same plane and aligned in the same direction are used.
Another configuration is described in the following article: “A High-Speed Parallel Robot for Scara Motions” Proceeding of the 2004 IEEE International Conference on Robotics & Automotion, New Orleans, La., USA, Apr. 26-May 1, 2004.
This robot is formed by four kinematic links, linked to the mobile platform, operated by four rotational actuators, linked to the fixed base. The main differences compared to the previously described configuration are the use of rotational actuators and the design of the mobile platform. Specifically, the mobile platform is formed by two parts linked together by means of a prismatic guide and a cable and pulley system transforming the linear translational movement of the guide into the desired rotation. This mobile platform design allows the actuators to be arranged at 90° in relation to each other, which involves a symmetrical design and a homogenous performance in the entire working volume.
The main drawback of the last two configurations described is the limitation to reach high accelerations and speeds, the main requirement for pick & place applications. This limitation is due to the use of prismatic guides and more specifically to the recirculation of the balls of the rolling guides.